Continuous ink jet (CIJ) printers typically consist of two main components, a fluid system and a print head or multiple print heads. Ink is pumped through a supply line from a supply reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the print head. Stimulations are applied to the print head to cause those ink streams to form streams of uniformly sized and spaced droplets, which are deflected into printing or non-printing paths. The non-printing droplets are returned to the supply reservoir via a droplet catcher and a return line. U.S. Pat. Nos. 4,734,711 and 5,394,177 and EP 1,013,450 describe in detail the design of a fluid system for CIJ apparatus. The more recent development of CIJ printing apparatus and print head fabrication can be found in U.S. Pat. No. 6,588,888 and U.S. Pat. No. 6,943,037.
Ink drop uniformity requires maintaining a uniform pressure in the print head cavity. U.S. Pat. No. 4,614,948 describes that a positive displacement pump, such as gear pump, is preferred for use as the ink supply pump. The need to limit pulsation produced by the pump is recognized in U.S. Pat. No. 4,971,527. In addition, filters are employed at appropriate locations in the fluid system to remove oversized particles prior to ink entering into print head orifices and avoid print head clogging.
Commercially available CIJ inks are mostly aqueous dye-based inks, where issues regarding robust system run-ability, such as easy start up/shut down, extended printing time without a crooked jet, and reduced frequency for filter changing have been minimized. In such traditional dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on coated glossy paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness and poor smear resistance.
Pigment-based inks have been proposed as a means of addressing limitations of dye based inks, particularly in drop-on-demand (DOD) inkjet printing systems. In pigment-based inks, the colorant exists as discrete particles. Pigment dispersions known in the art include self-dispersing pigment dispersions, dispersant stabilized pigment dispersions and encapsulated pigment dispersions. For non-self-dispersive pigments, the pigment particles are usually treated with addenda known as dispersants or stabilizers that serve to keep the pigment particles from agglomerating and/or settling out. There has been great progress in recent years in new dispersants/stabilizers for pigment dispersions. Materials are commonly classified into small surfactant molecules or polymers, containing both hydrophilic and hydrophobic segments. Small molecule surfactants disclosed in U.S. Pat. Nos. 5,651,813 and 5,985,017 are found useful due to their compatibility with a wide variety of pigments. Useful polymeric stabilizers include acrylic and styrene-acrylic random or block copolymers, such as those disclosed in U.S. Pat. Nos. 4,597,794, 5,085,698, 5,172,133, 5,821,283, 6,245,832, and 6,326,449. Polymeric dispersants have the additional advantage of offering image durability once the inks are dried on the ink receiver substrate.
However, it has been discovered that severe filter plugging issues may be encountered when running pigment inks in a CIJ fluid system, requiring frequent change of filters, over the time period of a few hours vs. a few months for dye-based inks. The consequence of filter plugging is the loss of fluid pressure and fluid jets, leading to system shutdown. Further investigation has shown that the gear pump commonly used in the fluid system to maintain fluid pressure with minimal pulsation can cause agglomeration of pigment particles, leading to filter clogging and system shutdown. Therefore, there exists a need for pigment ink formulations which are stable in CIJ fluid systems.
It is an object of this invention to provide an aqueous ink jet ink composition for a CIJ printing apparatus containing dispersed pigment, and to allow the ink to be run in a CIJ fluid system for extended periods without significant pressure build up or filter clogging.